Green aboveground shoots of the moss Polytrichum alpinum Hedw., growing on the tundra at Point Barrow, Alaska, were exposed to 14CO2 in the field. Autoradiography and subsequent counting revealed preferential translocation of the labeled compounds into new belowground shoots arising from the extensive underground stem system. Within 24 h all stem systems extracted had been labeled throughout, a maximum distance of 7.5 cm being covered in this time, indicating minimum rates of 3.1 mm h−1.Increases in the number of new shoots aboveground throughout the season revealed the extent and importance of this translocation. Innate markers of seasonal growth enabled ages of the shoots at two sites to be determined and also the ages of shoots at production of gametangia in a third site where fruiting was common. The relative photosynthetic rates of the successive seasons’ growth were also determined, and showed that by the 3rd year the photosynthetic capacity had dropped to about 40% of that in the youngest tissue on the same shoot.The patterns of growth and translocation of photosynthate in Polytrichum alpinum are discussed in relation to survival on the tunda.